Henrique BASTOS1,2, Nicolas SCHAEFFER2, Jennifer PRINGLE1, Joćo COUTINHO2, Cristina POZO-GONZALO1
1Institute for Frontier Materials, Burwood, Australia
2CICECO - Aveiro Institute of Materials, Aveiro, Portugal
The road to carbon neutrality largely relies on energy storage devices, especially lithium-ion batteries (LIBs). Their demand is projected to rise 14-fold by 2030, with over 11 million metric tons of LIBs reaching their end-of-life during this period. Spent LIBs require treatment to avoid leakage of toxic elements into the environment. They are also interesting sources of critical raw materials, such as lithium, nickel, cobalt, and manganese, due to the high purity grade of the elements in these matrices when compared to mineral ores. Spent LIB are conventionally recycled through pyro- and hydrometallurgy, with hydrometallurgy being potentially greener and more efficient when considering its lower energy requirements and gaseous emissions as well as its greater process flexibility. The economic and environmental credentials of the hydrometallurgical approach are dependent on the volume and corrosive nature of the generated waste effluent due to excessive acid consumption during the leaching stage. To improve the atomic efficiency of the leaching process, substitution of water as the reaction media by organic solvents, also called solvometallurgy, represents a promising approach.
Ionic liquids and their derivates, such as deep eutectic solvents (DESs), are considered as more selective metal leaching systems due to the varied effects in metal speciation. In DES, the combination of halide-based hydrogen-bond acceptors (HBA) and various hydrogen-bond donors (HBD) such as organic acids and alcohols have been shown as good candidates for efficient metal recovery and endorsed as reusable in multiple cycles, through chemical or electrochemical approaches. However, the contributions of each DES component in the leaching mechanism are often unclear. Moreover, their ‘green’ character has been challenged, with their high efficiency arising from decomposition products originated at typical operating conditions.
In this work, solvents based on alcohols, typical HBD components of DESs, and dilute HCl content have been used for the dissolution of Co, Ni and Mn oxides and their extraction from NMC. Apart from other effects such as acid concentration and water content, the contribution of the alcohols in the metal dissolution mechanism was assessed and shown to represent an important contribution. Alcohol-based media could increase chloride activity, a relevant ligand for metal dissolution, as well as act as a direct ligand for metals. Following identification of the most promising systems, these were used in an electrochemically-assisted leaching to further increase the efficiency of the process and reduce the amount of acid used. Results indicate that the leaching yield is higher in this electrochemical process compared to the non-redox chemical leaching. Based on the acidity, applied potential and temperature and Pourbaix diagrams of these metals in similar conditions, the selective leaching of metals from mixtures is expected to improve further. As reported for some DES, degradation products at extreme conditions with different origins than those in DES, could enhance metal leaching even further, despite mitigating its reuse.
This work can shed light on the use of non-aqueous ionic systems to enhance metal dissolution, opening the door for the design of simpler systems more compatible with existing metallurgical infrastructures.